Thermal Management of Electronics and Optoelectronics: From Heat Source Characterization to Heat Mitigation at the Device and Package Levels

Abstract

Thermal management of electronic and optoelectronic devices has become increasingly challenging. For electronic devices, the challenge arises primarily from the drive for miniaturized, high-performance devices, leading to escalating power density. For optoelectronics, the recent widespread use of organic light emitting diode (OLED) displays in mobile platforms and flexible electronics presents new challenges for heat dissipation. Furthermore, the performance and reliability of increasingly high-power semiconductor lasers used for telecommunications and other applications hinge on proper thermal management. For example, small, concentrated hotspots may trigger thermal runaway and premature device destruction.

Emerging challenges in thermal management of devices require innovative methods to characterize and mitigate heat generation and temperature rise at the device level as well as the package level. The first part of this dissertation discusses device-level thermal management. A thermal imaging microscope with high spatial resolution (~450nm) is created for hotspot detection in the context of diode lasers under back-irradiance (BI). Laser facet temperature maps reveal the existence of a critical BI spot location that increases the laser’s active region temperature by nearly a factor of 3. An active solid-state cooling strategy that could scale down to the size of hotspots in modern devices is then explored, utilizing energy filtering at carbon nanotube (CNT) junctions as a means to provide thermionic cooling at nanometer spatial scales. The CNT cooler exhibits a large effective Seebeck coefficient of 386μV/K and a relatively moderate thermal conductivity, together giving rise to a high cooling capacity (2.3 × 106 W/cm2).

Thermal management at the package level is then considered. Heat transfer in polymers is first studied, owing to their prevalence in thermal interface materials as well as organic devices (e.g., OLEDs). Employing molecular design principles developed to engineer the thermal properties of polymers, molecular-scale electrostatic repulsive forces are utilized to modify chain morphologies in amorphous polymers, leading to spin-cast films that are free of ceramic or metallic fillers yet have thermal conductivities as high as 1.17 Wm-1K-1, which is approximately 6 times that of typical amorphous polymers. Electronics packaging designs incorporating phase change materials (PCMs) are then considered as a means to mitigate bursty heat sources; PCM incorporation in a packaged accelerator chip intended for large-scale object identification is found to suppress the peak die temperature by 17%.